When left untreated, wood becomes subject to degradation by a variety of natural causes. The wood preservation process deposits or fixes chemical preservatives in the wood, and the toxic nature of the chemicals effectively prevents the attack of living organisms on the wood, preserving structural integrity and thereby extending the lifetime of the treated wood. In general, the wood preservation formulations desirably have the following characteristics: be toxic to attacking organisms; be able to penetrate wood; be chemically stable; be safe to handle; be economical to use; not weaken the structural strength of the wood; and not cause significant dimensional changes within the wood.
Traditional chemicals used for wood treatment are generally divided into two major groups: (i) organics—these are oil-borne chemical formulations, consisting of an organic preservative dissolved in a suitable petroleum oil carrier and (ii) inorganics—these are water-borne chemical formulations, consisting of inorganic compounds dissolved in water as a carrier. The most common wood preservatives include water-soluble inorganic metal salts that are supplied with or without co-biocides. These include Chromated Copper Arsenate (CCA), Ammoniacal Copper Arsenate (ACA), Alkaline Copper Quat (ACQ), Copper Azole (CA) and various forms of borates. These inorganic preservatives are inexpensive and do not require costly formulating agents. For example, CCA is completely water-soluble and requires no formulating agents, and alkaline copper based preservatives such as ACQ require only a complexing agent, such as ammonia or monoethanolamine, to ensure solubility of the copper component and to help chemical fixation in the treated wood.
Although Chromated Copper Arsenate (CCA), previously the most commonly used preservative, has been effective and economical, changing market perceptions and the public interest in alternative preservatives has led to changes in the treated wood products offered to consumers at retail. In consultation with the Environmental Protection Agency, manufacturers have made a transition to alternative wood preservatives for the residential and outdoor market. The wood treating industry voluntarily agreed to phase out CCA for residential applications at the end of 2003, although CCA is still approved for use in certain commercial applications. Residential applications include fencing, decks, picnic tables, playground equipment, and other construction projects that come into regular human contact in residential areas. Commercial applications include docks in salt or brackish water, boat construction, shakes and shingles, plywood flooring, laminated beams, highway barriers, agricultural timbers and poles, and similar projects.
As a result of withdrawing CCA for many applications, a renewed interest in the use of organic preservatives alone or in combination with co-biocides has occurred. These organic preservatives are highly effective fungicides having very low mammalian and aquatic toxicity. Due to their low solubility in water, organic biocides, excepting some quaternary ammonium compounds, are used in combination with organic solvents to form solutions, suspensions, or emulsions. Emulsions include conventional emulsions (macro-emulsions), micro-emulsions, or emulsifiable concentrates. To produce preservative emulsions, micro-emulsions, or emulsifiable concentrates, organic preservatives have been dissolved in organic solvents and mixed with various emulsifiers.
U.S. Pat. No. 4,954,338, European Patent Application 0148526, European Patent 0148526, and U.S. Pat. No. 5,536,305 disclose water dilutable formulations in which isothiazolone and triazole preservatives are dissolved in various organic solvents and mixed with emulsifiers.
In European Patent 0148526, commonly used emulsifiers such as ethoxylated phenol derivatives and ethoxylated oils are described as solubilizers for the active preservatives. The use of synthetic and natural resins and oils, such as drying oils, to improve the applicability and chemical/physical stability of the formulations is discussed.
WO 02/076696 discloses emulsifier free compositions containing a modified alkyd resin, solvent, preservative and water. In this disclosure, the preservative solution in organic solvent is emulsified by the acrylic acid modified alkyd resin, which serves as an emulsifier. The particle size and long-term stability of the resulting emulsion is not discussed.
Of concern regarding the use of organic preservatives is the limited stability of suspensions and the need for organic solvents, which are expensive, difficult to handle due to their flammable or combustible nature, and are environmentally sensitive due to volatile organic compound (VOC) emissions and potential toxicity.
An additional concern in the use of organic preservatives is fixation and depletion. While most inorganic preservatives are “fixed” in the treated wood through a combination of chemical reactions, physical interactions, and precipitation, most organic preservatives have limited chemical and physicochemical interactions with wood. Although most organic preservatives have low water solubility, normally in the part per million (PPM) range, preservative depletion is significant under in-service conditions. It has been estimated that certain organic preservatives could lose more than 20% of the original loading after only one year of aboveground field exposure (F. Cui, 2002, unpublished results). Another concern when using organic preservative compositions is a poor distribution gradient that is unsuitable for preventing decay. Some organic preservatives can have such a steep distribution gradient that the center of the treated wood does not have adequate preservative loading to prevent decay, even though the outer layer has more than adequate preservative loadings. In contrast, inorganic preservatives such as CCA do not usually have prohibitive distribution gradients at concentrations typically used, i.e. the chemical concentration difference in the outer and inner layers of the treated wood is insignificant.
The use of acids in wood preservative formulations has been described in the patent literature. For example, EP 0 402 697 describes mixtures of fenpropimorph and water-insoluble acids. Mixtures of amines and acids are described in DE-A-3 736 298. U.S. Pat. No. 5,880,143 describes triazole based compositions containing tertiary amines and dicarboxylic acids in the presence of optional monocarboxylic acids such as 2-ethylhexanoic acid. The main purpose of using the carboxylic acids, in combinations with amines, seems to be corrosion resistance and formulation stability. In addition, U.S. Pat. No. 5,880,143 requires the use of water-soluble or water miscible organic solvents. Carboxylates are used as counter ions for quaternary ammonium compounds in U.S. Pat. No. 5,013,748. U.S. Pat. No. 5,013,748 describes compositions containing at least one triazole, a benzimidazole, and at least one quaternary ammonium compound dissolved in an ether type solvent as well as a carboxylic acid in the presence of optional oils and fixatives. Acids described in U.S. Pat. No. 5,013,748 included formic acid, acetic acid, and propionic acid. The purpose of these low molecular weight acids is not discussed. Sustained release antifungal formulations containing polyacrylic acid and triazole derivatives are described in U.S. Pat. No. 6,423,307. Polyacrylic acid is described as a bioadhesive to improve bioavailability of the active fungicide.
Solvent-free compositions based on quaternary ammonium compounds (U.S. Pat. No. 6,464,764) and amine oxides as solubilizers (WO 00/71314, WO 00/71313) have been described. Tests (unpublished results) suggested that the use of high levels of fungicidally active quaternary ammonium compounds did not significantly contribute to the efficacy of triazole type organic preservatives. In addition, the presence of quaternary ammonium compounds poses compatibility problems with most emulsion water repellents. Quaternary ammonium compounds and amine oxides require long hydrocarbon chains to provide water repellency. Formulations containing these long chain amine oxides or quaternary ammonium compounds are difficult to handle due to their high viscosity.
Drying oils (U.S. Pat. No. 6,641,927) and oxidatively drying polybutadiene polymers containing quaternary ammonium functions (U.S. Pat. No. 4,496,613) have been described for preservative compositions. In U.S. Pat. No. 6,641,927, neat soybean oil was described for use to impregnate wood. See also U.S. Pat. No. 6,686,056; U.S. Publ. Appl. No. 2005/003190; and U.S. Publ. Appl. No. 2003/0108759 disclosing reactive oil/copper preservative systems for wood products.